Embodiments of the present invention relate to an electrostatic chuck for holding a substrate in a process chamber.
In the processing of substrates, such as semiconducting wafers and displays, a substrate is placed on a substrate support in a process chamber and suitable processing conditions are maintained in the chamber, for example, to etch or deposit material on the substrate. The support can include an electrostatic chuck having at least one electrode that can be electrically charged to electrostatically hold the substrate on the support. The electrode can also be electrically biased, for example with a high frequency electrical power, such as RF (radio frequency) power to energize process gas provided in the chamber to process the substrate. The support typically includes a pedestal that supports the electrostatic chuck to provide better temperature control of the chuck and to allow raising and lowering of the height of the chuck in the chamber.
The temperature of each substrate has to be precisely controlled to ensure temperature uniformity across the substrate surface; otherwise, uneven etch and deposition rates are obtained across the substrate. It is also necessary to maintain uniform temperatures from one substrate to another in the processing of a batch of substrates. Substrate temperature control is achieved by controlling the thermal impedance properties of the contact surface between the chuck and the substrate. The temperature of the substrate can also be controlled by supplying a heat transfer gas, such as helium, to the backside of the substrate.
However, it is difficult to precisely control substrate when the surface texture of the contact surface of the chuck is uneven or has an uncontrolled surface roughness. The surface texture comprises peaks and valleys that vary in height and spacing depending on upon the way the surface was produced, for example, machine surfaces have peaks and valleys with uniform spacing and direction while ground surfaces have a more random spacing. The uneven surface texture of the contact surface can result in different thermal resistances across the backside of the substrate causing different temperatures across the substrate frontside. Also, an uneven contact surface can result in leakage of the heat transfer gas from selected regions at the interface of the substrate and the contact surface. Leaking of the heat transfer gas reduces the pressure of the heat transfer gas against the substrate backside causing loss of heat transfer efficiency and corresponding rises in temperature at those selected regions of the substrate.
Substrate surface temperature variations have been reduced by polishing the contact surface to, for example, have a predefined range of surface roughness average. The chuck surface can be polished by, for example, rotating a grinding pad against the surface until the surface has a defined surface roughness average. Controlled surface roughness reduces leakage of heat transfer gas because the relatively smooth surface is pressed substantially flush against the backside of the substrate to seal the heat transfer gas between the substrate and chuck. The surface roughness values can be measured using conventional profilometers.
However, even chucks having polished contact surfaces with low surface roughness values can give rise to unexplainable substrate temperature variations. Local surface non-uniformities also result in poor chucking because variations in surface texture affect the electrostatic chucking and the time required to de-chuck the substrate. The local surface variations and anomalies become particularly problematic in the processing of features having smaller and smaller dimensions, for example, interconnect lines having widths of less than 90 nm. Also, when processing a batch of substrates to etch such lines, it was found that the later substrates processed on the chuck surface were often found to have more variation in etching rates and different critical dimensions (CD) than earlier processed substrates. These chucks were simply removed from the chamber and scrapped because the cause of the change in processing characteristics was not known. The undesirable shift in etching rates and CD also makes it difficult to select proper process parameters to achieve consistent substrate processing results for a batch of substrates. Thus, conventional polished chuck surfaces having selected range of surface roughness average often failed to provide consistent processing results for a batch of substrates.
Thus, it is desirable to have a substrate support and electrostatic chuck having a contact surface that provides good substrate temperature control and consistent processing results for processing a batch of substrates. It is furthermore desirable to have a method of selecting or fabricating the substrate support, which allows better control of surface anomalies that result in inconsistent processing results. It is further desirable to be able to reuse a chuck, which has been used to process a number of substrates without scrapping the chuck.